1. Field of the Invention
The present invention generally relates to the processing of information signals, such as audio signals, video signals or other multimedia signals, and particularly to the processing of information signals in the spectral/modulation spectral range.
2. Description of the Related Art
In the field of signal processing, such as the processing of digital audio signals, there are frequently signals consisting of a carrier signal component and a modulation component. In the case of modulated signals, a representation in which the signals are decomposed into carrier and modulation components is often required, for example to be able to filter, code or otherwise modify them.
For the purposes of audio coding, it is known, for example, to subject the audio signal to a so-called modulation transform. Here, the audio signal is decomposed into frequency bands by a transform. Subsequently, a decomposition into magnitude and phase is performed. While the phase is not processed any further, the magnitudes per subband are re-transformed via a number of transform blocks in a second transform. The result is a frequency decomposition of the time envelope of the respective subband into modulation coefficients. Audio codings consisting of such a modulation transform are, for example, described in M. Vinton and L. Atlas, “A Scalable and Progressive Audio Codec”, in Proceedings of the 2001 IEEE ICASSP, 7-11 May 2001, Salt Lake City, United States Patent Application US 2002/0176353A1: Atlas et al., “Scalable And Perceptually Ranked Signal Coding And Decoding”, Nov. 28, 2002, and J. Thompson and L. Atlas, “A Non-uniform Modulation Transform for Audio Coding with Increased Time Resolution”, in proceedings of the 2003 IEEE ICASSP, 6-10 April, Hong Kong, 2003.
An overview of further various demodulation techniques across the full bandwidth of the signal to be demodulated including asynchronous and synchronous demodulation techniques, etc. is given, for example, by the article L. Atlas, “Joint Acoustic And Modulation Frequency”, Journal on Applied Signal Processing 7 EURASIP, pp. 668-675, 2003.
A disadvantage of the above schemes for audio coding using a modulation transform is the following. As long as no further processing steps are performed on the modulation coefficients together with the phases, the modulation coefficients form a spectral/modulation spectral representation of the audio signal that is reversible and perfectly reconstructing, i.e. it is re-convertible without changes back into the original audio signal in the time domain. However, in these methods the modulation coefficients are filtered to reduce and/or quantize the modulation coefficients to values as small as possible according to psychoacoustic criteria, so that a maximum compression rate is achieved. However, this generally does not accomplish the desired goal to remove the respective modulation components from the resulting signal or to deliberately introduce quantization noise in this component. This is due to the fact that, after the back-transform of the changed modulation coefficients, the phases of the subbands are no longer consistent with the changed magnitudes of these subbands and continue to contain strong components of the modulation component of the original signal. If the phases of the subbands are now recombined with the changed magnitudes, these modulation components are reintroduced into the filtered or quantized signal by the phase. In other words, a modulation transform followed by a modification of the modulation coefficients in the above manner, i.e. by filtering the modulation coefficients, together with a subsequent synthesis of the phase and magnitude components provides a signal that, in another analysis and/or modulation transform, still contains significant modulation components at those places in the spectral/modulation spectral range representation that should have been filtered out. Effective filtering is thus not possible based on the above-mentioned modulation transform-based signal processing schemes.
Therefore, there is a need for an information signal processing scheme allowing to process modulated signals with a carrier component and a modulation component separated according to modulation and carrier component in a more controlled way.